The present invention relates to an optical sensor and solid-state imaging device, and specifically, to a one-dimensional or two-dimensional solid-state imaging device of CMOS or CCD sensors, and an operating method for the above-described solid-state imaging device.
Image sensors such as a CMOS (complementary metal-oxide semiconductor) image sensor and CCD (charge coupled device) image sensor have been improved in their properties and find widespread application in digital cameras, cellular phones with cameras, scanners, and so on.
However, further property improvements are demanded of the image sensors. One of them is to extend the dynamic range. The dynamic range of conventionally used image sensors remains, for example, on the order of 3 to 4 digits (60 to 80 dB), and hence, expectations are being placed on the realization of a high-quality image sensor having a dynamic range at least 5 to 6 digits (100 to 120 dB), comparable to that of the naked eye or a silver-halide film.
As a technique for enhancing the image quality of the above-described image sensors, for example, S. Inoue et al., “IEEE Workshop on CCDs and Advanced Image Sensor 2001, pp. 16-19” (hereinafter referred to as “Non-Patent Document 1”) sets forth, with a view to bringing about a high sensitivity and high S/N ratio, a technique for reducing noises by reading noise signals occurring in a floating region adjacent to the photodiode of each pixel and the noise signals added with an optical signal, and taking the difference therebetween. However, even by this method, an achievable dynamic range would be on the order, at the highest, of 80 dB. Realization of even wider dynamic ranges is demanded.
Also, for example, as shown in FIG. 1, Japanese Unexamined Patent Application Publication (JP-A) No. 2003-134396 (hereinafter, “Patent Document 1”) discloses a technique for extending the dynamic range by connecting a floating region having a small capacitor C1 located on the high-sensitivity and low illuminance side, as well as a floating region having a large capacitor C2 located on the low-sensitivity and high illuminance side to the photodiode PD, and outputting a low-illuminance side output OUT1 and a high-illuminance side output OUT2, respectively.
Furthermore, as shown in FIG. 2, Japanese Unexamined Patent Application Publication (JP-A) No. 2000-165754 (hereinafter, “Patent Document 2”) discloses a technique for extending the dynamic range by making variable a capacitor CS in the floating diffusion (FD) region. Moreover, there is another disclosed technique for extending the dynamic range by dividing an imaging into imagings with at least two different exposure time periods, including an imaging with a short exposure time period corresponding to the high illuminance side and an imaging with a long exposure time period corresponding to the low illuminance side.
Furthermore, as shown in FIG. 3, Japanese Unexamined Patent Application Publication (JP-A) No. 2002-77737 (hereinafter, “Patent Document 3”), and Y. Muramatsu et al., IEEE Journal of Solid-State Circuits, Vol. 38, No. 1, pp. 16-19 (hereinafter, “Non-Patent Document 2”) disclose a technique for extending the dynamic range by providing a transistor switch T between a photodiode PD and a capacitor C, turning on the switch T in a first exposure period to store optical signal charges in both of the photodiode PD and capacitor C, and turning off the switch T in a second exposure period to store optical changes in the photodiode PD on top of the stored charges in the first exposure period. Here, these documents disclose therein that, when there is provided light irradiation beyond its saturation value, excess charges are discharged through a reset transistor R.
Furthermore, as shown in FIG. 4, Japanese Unexamined Patent Application Publication (JP-A) No. 5-90556 (hereafter, “Patent Document 4”) discloses a technique for allowing addressing high-illuminance imaging by using a capacitor larger than conventional one, as a photodiode PD.
Besides, as shown in FIG. 5, The Journal of the Institute of Image Information and Television Engineers, Vol. 57, 2003 (hereinafter, “Non-Patent Document 3”) discloses a technique for extending the dynamic range by storing and outputting optical current signals from a photodiode PD while logarithmically converting the signals by a logarithm conversion circuit constituted by combining MOS transistors.
In the methods set forth in the above-described Patent Documents 1, 2, and 3, and Non-Patent Document 2, or the method for imaging with two or more different exposure time periods, the imaging on the low illuminance side and the imaging on the high illuminance side are performed at times different from each other. This raises a problem in that a time lag occurs between imaging times of the at least two imagings to thereby impair qualities of moving images.
Moreover, in the methods set forth in the above-described Patent Documents 4 and 3, a wide dynamic range can be achieved by the imaging such as to correspond to the high-illuminance side, but as far as the imaging on the low-illuminance side is concerned, a low sensitivity and low S/N ratio are undesirably resulted to thereby impair image quality.
As described above, in image sensors such as the CMOS image sensor, it has been difficult to attain a wide dynamic range while maintaining a high sensitivity and high S/N ratio. The foregoing applies not only to image sensors in which pixels are arranged in a two-dimensional array, but also to linear sensors in which pixels are arranged in a one-dimensional array, and an optical sensor without a plurality of pixels.